Seed potatoes

by Tamara Scully
Although there have been recent efforts to grow potatoes directly from true seed, rather than tubers, the commercial potato industry relies on tuber seed potatoes to maintain disease-free planting stock. Growing from tubers is a type of cloning, as the progeny has the same genetic makeup of the parent.
When tubers are grown in the field, both the tubers and the soil accumulate pathogens. Therefore, the number of generation of progeny is limited in certified seed production although this varies by certifying agency. Many states have their own certified seed potato services. Seed certification is handled either by a land-grant university, a state department of agriculture, or a growers’ association.
Many potato viruses are carried in the tubers, and can dramatically cut yields. Insuring disease-free planting stock through careful production and testing is the industry’s primary defense against diseases.
“Isolation is a seed farmer’s best friend,” Jim Gerritsen, who farms with his family at Wood Prairie Farm in Bridgewater, Maine, said in a recent webinar. Gerritsen, an outspoken advocate of organic farming practices, predicts that isolation will become even more important.
Clean stock
If seed potatoes are infected with diseases or insects, many of which are difficult to eradicate, crop yield and quality is impacted.
When selecting seed potatoes, “you need to be careful because what you are bringing into your soil is something that will have long-lasting harm,” he said.
Certified growing stock is first tested and determined to be free of any disease concerns. Plants are propagated via tissue culture in the laboratory from disease-free stock. Disease-free shoot tips are collected and planted in vitro. Sprouts are grown for three weeks then cuttings are taken from each. In this way, the seed potato is rapidly multiplied, and large quantities of disease-free stock can be produced.
When shoots of the potato plant are grown in the laboratory, they are “growing faster than a potato virus can grow,” Gerritsen said. “All modern day potatoes grown are the result of tissue culture.”
The shoots, or stem cuttings, are then transplanted into a soil-less medium. Traditionally, this has been a vermiculite mix. Recently, the industry has begun using nutrient film in a “hybrid-hydroponic” growing environment, where a solution of synthetic feed is constantly misted over the roots.
In either method, the shoots grow into plantlets, which produce mini-tubers. Mini-tubers can be harvested at a variety of sizes, from that of a pencil eraser to about the diameter of a quarter, Gerritsen said. Utilizing nutrient film allows mini-tubers to be harvested whenever they reach the desired size, rather than harvesting the entire plant at once to get to the tubers. This increases mini-tuber yield, and has become the industry standard.
Depending on production practices and finances, farmers select the size mini-tuber that best works in their growing situation. At Wood Prairie, where the growing season is short, Gerritsen selects nickel or dime-sized mini-tubers, allowing for an earlier planting time than smaller sizes.
“The smaller the tuber, the longer they take to emerge,” he said. In an organic system, where the plants need to be harvested by early August to avoid disease transmission by aphids, and where weed pressure is a concern, a somewhat larger size is important, although it costs more.
Field planting
Mini-tubers are then planted into the fields. Prior to planting, potatoes grown at Wood Prairie Farm are subjected to a modified green sprouting technique, where they are held for a week in warm temperatures.
Once planted, the fall-harvested tubers are considered the next generation. Certified seed growers limit the number of generations grown, to decrease disease risks.
With over 20 varieties of seed potatoes and three generations of each on the farm each season, proper labeling and maintaining separation of the tubers is essential. Each January, Gerritsen determines which tubers are the best lot for ongoing seed stock production, or which are best to sell to customers who will then harvest and use the potatoes for food. Stock is tested, and as a grower there needs to be less than 0.5 percent of a virus presence, he said.
“Every year we renew mini-tubers so that we are always starting fresh,” Gerritsen said. “It can get a little bit confusing.”
Green sprouting
When planting potatoes, chitting or green sprouting is “an optional pre-conditioning step in growing the crop,” which helps it to begin growth more rapidly, Gerritsen said. It is a “very intensive technique,” which increases yield, and is a two-step process.
“We green sprout virtually all of the seed we plant. It’s well-suited to an organic production system,” he added. Green sprouting cuts 10-14 days off the growing cycle, by causing the seed potatoes to be “primed and ready to grow faster.”
With 25,000 pounds per year of seed potatoes planted at Wood Prairie Farm, green sprouting is an added step that helps to insure that late season varieties are mature enough to yield well, that drought won’t be as detrimental to the crop, and that tuber yield is increased.
Each potato variety has its own dormancy requirements. Some are slow to break dormancy, and green sprouting is especially useful for these, particularly in short season growing areas, like Maine.
The crop is held for 7-10 days at temperatures of 70-75 degrees F, in the dark, until the beginnings of the sprouts are seen in the eyes. The tuber is then exposed to light, either artificial or natural, while the temperature is dropped to 50 degrees. The light needs to be “sufficient and strong” enough to elongate the sprout in a concentrated manner. The drop in temperature is needed to maintain energy in the seed. Respiration will increase moisture loss, and deplete the tuber’s energy.
“You’re trying to manage respiration at a rate which will maintain the greatest amount of energy in that seed tuber,” Gerritsen said. “Energy is directly connected to potential for yield.”
Green sprouting increases the number of stems by suppressing the growth of the dominant apical stem, which will grow a large potato at the expense of yield. Each eye sprout on the potato will become a stem. Green sprouting will increase sprouting, increase the number of stems, and increase yield. It also increases tuber “set,” which occurs about four weeks after emergence.
Tuber set requires adequate moisture, whether from rain or irrigation. There is about a 10-day window to get the moisture to the tubers, Gerritsen said. Green sprouting will increase overall yield, but the yield will still be impacted by dry conditions during tuber set.
“If everything else is going right then you can gain the highest possible yield,” with green sprouting, Gerritsen emphasized.
For farmers wishing to have the earliest potato crops, green sprouting is non-negotiable. Whether to assist with disease prevention and short growing seasons, or to gain customer loyalty by having the product sooner than other growers, green sprouting provides an advantage.
Gerritsen, a certified seed potato producer, has been hosting a series of educational webinars on seed potato production on the techniques required to produce clean seed potatoes, and why this is so important.